Broad-area ion sources are widely used in vacuum-based materials processing applications. Such ion sources can include ion beam extraction and acceleration grid optics to produce an ion beam with a relatively narrow range of ion energy. The ion sources often have features to help control spatial and charged-state attributes of the ion beam such as beam current density or flux, uniformity or spread, divergence or focus, and neutralization. Broad-area ion sources also include gridless ion sources such as Hall-current or closed-drift ion sources, anode-layer ion sources, and so-called end-Hall ion sources. These ion sources typically use a magnetic field to assist an accelerating space charge in proximity to an anode that accelerates ions with typically higher current fluxes than that achieved with gridded ion sources. Such sources are differentiated from small aperture ion beam sources such as those used in analytical spectroscopy work, high-energy physics or high-energy ion implantation.
Broad-area ion beams serve a wide variety of terrestrial materials processing applications including ion milling, surface etching, texturing, and pre-cleaning in order to pattern, produce, or “grow” various surface structures or alter surface properties. Such ion sources are also used in a host of direct or indirect thin film deposition processes including ion beam physical vapor sputter deposition, direct film deposition (e.g., silicon-carbon based or diamond-like carbon coatings), or as an energetic ion assist to magnetron, e-beam evaporator, or secondary ion beam deposition processes for the formation of either conductive or dielectric films. An example of a broad-area ion beam assist process may include the electron-beam deposition of silicon-, aluminum-, tantalum-, or titanium-bearing compounds onto an optical substrate while impinging the same substrate with an O2 ion beam from either a gridded or gridless ion source in order to produce films with high optical index and clarity and with controlled material properties such as adhesion, stress, or density. Another example of the use of broad-area ion beam for materials processing may include the use of a linear gridded or gridless ion source for surface treatment of moving architectural glass substrates undergoing vacuum-based processing, or metal or polymer films in a vacuum web-coating system. In such instances the broad-area ion beam(s) can be used for surface cleaning, texturing, deposition assist, surface chemistry activation, or the formation or growth of nano-structures.
Most gridded and gridless ion source work by introducing a gas or evaporated compound into the ion source body and forming an electrically conductive gas discharge or plasma in order to derive charged ion species and states. Special power supplies are often required to deliver and control the level of power into the gas discharge to maintain a stable state and concentration of charged-particles (or ions). However, the steady state performance and beam output of the ion source is also highly dependent upon other input and system state factors such as the input gas flow, magnetic fields, electrostatic fields, electron emission element states, localized gas pressure(s), the net power density disposed into the gas discharge, and any emission or neutralization currents used to control volume or surface charging. Furthermore, the relation of the source output to any input factor can be influenced by the environmental factors such as the condition of the vacuum chamber, working geometry scale, emission of materials, or temperature and conductivity of wall boundaries. As such, the working states and environment of ion beam sources are not necessarily fixed nor is the relationship between input and output states time-invariant or independent of the working environment. For this reason, it is often necessary to include closed-loop control and performance regulation features into the operation of broad-area ion sources to maintain a consistent operating state of the ion source and output ion beam properties when used in manufacturing.
A “closed-loop” system generally refers to a system that takes a signal from an output and feeds the signal to an input (e.g., a feedback loop) to correct system performance. With respect to an ion source, examples of important input variables include gas flows, vacuum pressure, power (currents and voltages), magnetic fields, and reference voltages or currents such as primary or secondary emission currents. Examples of output variables include a measure of the ion beam current (or current density) directly or, as in most cases, the discharge current, voltage or impedance state, and any neutralization current. As in any control system, the system state relating inputs and outputs can be characterized by some transfer function that governs amplitude and temporal states of the system. For closed-loop control, the system also requires a feed-back loop that can be characterized by a feed-back function, mechanism, circuit, or algorithm that incorporates closed-loop control parameters (i.e., gain, delay, or phase). The system can be a single variable system (i.e., single input and output) or multivariate system. (A more detailed reference on the analysis, design and application of control systems is available in Ellis, Control System Design Guide, Academic Press (1992). Also, details on the analysis and application of multivariate closed-loop control and feedback conditioning is available in Skogestad and Postlethwaite, Multivariable Feedback and Control, Analysis and Design, John Wiley and Sons (1996).
The current closed-loop control systems commonly used for broad-beam ion sources utilize feedback to one or two control variable inputs such as input gas flow and certain electron emission current or input voltages. In these known implementations, the control systems use fixed, closed-loop parameters that must serve the wide domain of the operation, scale, and installation environment of the ion source and unknown, time-variant behaviors for different process conditions. If the parameters need to be varied to improve dynamic operation, empirical work must be performed to identify and verify the efficacy of any new closed-loop parameters.